Plasma cell voltage control circuit

ABSTRACT

The present circuit controls the width of the voltage pulses applied to a plasma cell to control the amount of wall charge that builds within the cell. By controlling the level of the wall charge it is possible to control the occurrence of the discharge within the cell.

United States Patent Skutt 1451 Apr. 2, 1974 [5 PLASMA CELL VOLTAGECONTROL 3,600,631 8/1971 Cake 315/169 T x CIRCUIT 3,227,922 1/1966Glaser et al 313/1095 X 2,906,906 9/1959 McCauley et al 313/1095 [75]Inventor: Robert R. Skutt, Kettering, Ohio [73] Assignee: The NationalCash Register Primary Examiner Alfred L Brody Company Dayton OhmAttorney, Agent, or Firm-J. T. Cavender; Albert L.

[22] Filed: Nov. 6, 1972 Sessler, Jr.; Edward Dugas 21 Appl. No.:303,834

57 ABSTRACT [52] US. Cl. 315/169 TV, 313/109.5, 315/270 1. [51] 1t. Cl.H051) 41/16 The present circuit controls the width of the voltage 58 116111 61 Search 315/169 TV, 169 T, 270, pulses applied to a plasma cellto control the amount 315/271; 313/1095 107's of wall charge that buildswithin the cell. By controlling the level of the wall charge it ispossible to control [56] References Cned the occurrence of the dischargewithin the cell.

UNITED STATES PATENTS 3,614,769 10/1971 Coleman 315/169 TV X 7 Claims, 8Drawing Figures 20 Q L ,KEEP 1 --f r LIVE 1 KEYBOARD 1 s DRIVER 1': ELLl l AMPS 1 1 1 fi A AMP +54 1 l l B AMP H 1 TIME DATA #OR C AMP 1 I 111' 1 DELAYS CONTROLS KEYS i 95 9 AMP 1, 1 1 21 23 2e 1 E/E -95 E AMP:;1l 1 1 N 1 1 i F 2 F AMP 16' os'c DELAY 3 W i 1 *DR 6 AMP l I CLOCK TSHOT I ERASE KEY: l

1 .1 -1 40 6| I E T S 4| OR 62\ AMP I SHOT K AMP 1.1 24 27 42 1 i R E' HAMP I R I DELAY ONE L l I 2ST SHOT I 1 25 28 l E E 1 DELAY ONE I 1 36TSHOT 1 1 1 PLASMA CELL VOLTAGE CONTROL CIRCUIT BACKGROUND OF THEINVENTION Plasma (gaseous) displays utilizing the phenomenon of wallcharge have been used in the past. In Great Britain Pat. No. 1,161,832entitled Gaseous Display and Memory Apparatus there is disclosed aplasma gas cell having electrodes which are insulated from the gas, andwhich electrodes are connected to a source of pulsing discharge signalsfor forming a wall charge in the cell. The presence or absence of thewall charge inparts information similar to a memory. For example whenthe wall charge is present within the cell it is possible to cause adischarge in the gas using a potential having a level as low as one-halfof the level required to cause a discharge in the absence of a wallcharge. Interrogation of the cell with a potential 'level equal toone-half the level required for a discharge, or a level slightly lessthan the level of the potential used to form a wall charge, will causethe cell to discharge if the wall charge is present and not to dischargeif it is not present.

In the referenced patent the existence or nonexistence of a wall chargeis the information element. The present application is directed to acircuit for controlling the level of formed wall charge by utilizing apulse width circuit to vary the pulse width between different finiteincrements of width so as to control the formation time allowed to thewall charge during each cycle of pulses applied across a cell. When thelevel of the wall charge is added to the applied pulsed signal adischarge will occur if the sum of the levels is above the ionizationpotential level of the gas used in the cell. If the cell is in adischarge condition and the level of the wall charge as added to theapplied pulse signal is not below the extinguishing level of the cellthe discharge will continue to exist. Once the level of the combinedwall charge and applied pulsed signal falls below the extinguishinglevel of the cell the discharge ceases until the firing level is againexceeded.

SUMMARY OF THE INVENTION In the present invention the amount of wallcharge which is allowed to form within a plasma gas cell is controlledby controlling the pulse width of the signals applied to the cellelectrodes.

In the preferred embodiment of the present invention there is provided aclock source for providing a train of pulses, which train of pulses arefed to a plurality of delay means for delaying the pulsed signals fortime periods corresponding to the width of the pulsed signals. Acombining means combines selected delayed pulse signals with theprovided pulse signals to form a composite pulse signal having thedesired pulse width. Means are provided for applying the combinedsignals first to one electrode of the plasma gas cell and then to theother electrode. When the combined signal is not being applied to anelectrode the electrode is maintained at a ground potential. Akeep-alive cell, wherein a continuous discharge is taking place isplaced in coupling proximity to the gas of the plasma cell to provideionized particles. A plurality of plasma cells can be used with switchesselecting which cells are to receive the composite pulsed signals forcausing a plasma discharge to occur in the cell.

From the foregoing it can be seen that a primary object of the presentinvention is to provide an improved means for controlling the dischargewithin a plasma gas cell.

It is a further object of the present invention to provide a means forcontrolling the magnitude of a wall charge within a plasma gas cell.

It is another object of the present invention to provide a circuit for adigital pulse width control of the discharge in a plasma gas cell.

These and other objects of the present invention will become moreapparent when taken in conjunction with the following description anddrawings, wherein like numbers indicate like parts and which drawingsform a part of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block schematic diagram ofthe preferred embodiment of the present invention;

FIG. 2 is a schematic representation of the cell electrode configurationused in the embodiment of FIG. 1;

FIGS. 3a to 3c are graphs illustrating operating conditions for a plasmacell; and

FIGS. 4a to 4c are wave forms useful in understanding the operation ofthe embodiment shown in FIG. 1.

DISCLOSURE OF THE INVENTION In FIG. 1 a single character plasma gasdisplay device 10 is shown connected to the discharge control circuit 20of the present invention. The plasma cell can be of the type shown inUS. Pat. No. 3,614,769 entitled Full Select-Half Select Plasma DisplayDriver Control, by William E. Coleman et al., which patent is assignedto The National Cash Register Company of Maryland, the Assignee of thepresent application. The single character plasma display device is showncomprised of a common electrode 11 and seven individual electrodes, A toG, spaced from the common electrode. The combination of an individualelectrode and the common electrode will be called a cell. In thepreferred embodiment the individual electrodes are positioned in thealpha-numeric configuration shown in FIG. 2. A plasma gas occupies thespace between the electrodes and each of the electrodes is conductivelyinsulated from the gas. When an ionizing potential is applied across thecommon electrode and an individual electrode a discharge takes placebetween the electrodes which discharge allows a charge to build on thedielectric covering the c'oncerned electrodes. The charge is of such apolarity as to oppose the applied potential thereby causing theeffective potential across the gas to decrease in value below the levelthat would sustain a discharge. The sustaining potential level issubstantially lower than the ionizing level, provided that it is of thesame polarity as the previously established wall charge. Therefore oncea discharge takes place the potential necessary to maintain thatdischarge will be less than the potential necessary to start it. Tomaintain a discharge in the face of a wall charge it is necessary toalternate the potential as applied to the electrodes. In thisalternating (reversing) process the wall charge is added to the appliedpotential to achieve the ionizing potential. When the applied potentialis reversed the wall charge decreases from its former value and startsto build in the opposite direction. By controlling the time period(width) of the applied potentials across the elec trodes it is possibleto control the magnitude to which the wall charge may build such that onthe next cycle of reversing applied potentials the sum of the wallcharge and the applied potential does not reach a sufficient level tocause or sustain a discharge. On the other hand control can be such asto allow the summed potentials to exceed the discharge potential toachieve a discharge or to maintain one. The width and magnitude of theapplied electrode potential which will cause a discharge is a functionof the gas pressure, mixture and physical characteristics of the cell.Typically the gas is a mixture of neon, nitrogen, and argon.

To insure the presence of ionized particles in the vicinity of aselected cell there is provided in the plasma device a keep-alive cellcomprised of the two electrodes 13. Sustaining potentials arecontinuously applied in an alternating cycle across the keep-aliveelectrodes during the operation of the device. Although only a singlecharacter display is shown it will be obvious that a multi-characterdisplay could be operated in a similar manner utilizing the teachings ofthe aforementioned assigned patent and the present application.

Referring to FIG. 3a the voltage level V F denotes the potential levelabove which a plasma cell fires. A potential above the lower levellabeled V x but below the level V will sustain the cell in a dischargedcondition but will not initially cause a discharge. The level labeled Vis the level below which the cell is extinguished. In FIG. 3a thepotential level of the pulses applied to the electrodes is set at V,,.Initially the potential differential appearing across the commonelectrode and an individual electrode is limited to the level of theapplied potential, that is V,. At that point in time the wall charge inthe cell is equal to zero. Because the keep-alive cell, once discharged,is continually in operation there is available near the common andindependent electrodes a plurality of ionized particles. The potentialV, acts on these particles to start a wall charge build up on opposingsurfaces of the cell. Alternating the application of V, first to theindependent electrodes and then to the common electrodes causes thepreviously formed wall charge to be added to the potential V,. Continuedapplication of the V, pulses forces the wall charge to reach a level ofsufficient magnitude such that combining it with the level of the Vpulses results in a potential level that exceeds the firing level forthe cell.

This condition is shown in FIG. 3b. Once the cell has been dischargedthe potential necessary to maintain the discharge is less than thefiring potential. This being the case the wall charge can be reduced toa level which just maintains the combination of potentials above theextinguishing level V The charts of FIGS. 3a, 3b and 3c are typical,specific values not being given because of variations in the valuescaused by the physical configuration of the plasma cell, the compositionof the gas, the gas pressure and the pulse repetition rate of theoscillator clock. Once the cell has been constructed, the voltage levelsat which the cell fires and extinguishes for a given pulse width can besimply determined by experimentation.

Now referring back to FIG. 1 and specifically to the control circuit anoscillator clock 21 provides a train of pulses having a repetition rateof approximately 50 microseconds. The train of pulses is fed undelayedto a one shot multi-vibrator 22, the output of which is a pulsedesignated S having a pulse width of approximately 0.5 microsecond. Thetrain of pulses is also fed to the delay circuits 23, 24 and 25 forcorresponding delays ofT, 2ST and 26T, where the time T is equal to 0.5microsecond. The output pulses from the delay circuits 23, 24 and 25 arefed to the inputs of one shot muIti-vibrators 26, 27 and 28,respectively. The output pulses W, R and E from the multi-vibrators alsohave a width of 0.5 microsecond.

The undelayed pulses S are fed as one input to the bank of OR gates 30and to the input of the keep-alive OR gate 40. The delayed pulses W arefed as the other input to the bank of OR gates 30 through keyboard 50,comprised of data keys controlling switches 51.

The delayed pulses W are also fed as the other input to OR gate 40. Thedelayed pulses R are fed as an input to OR gate 41, and through an erasekey (when acti vated) to the input of an OR gate 42. The delayed pulsesE are fed as inputs to OR gates 41 and 42.

The bank of OR gates 30 correspond in number to the number of individualelectrodes, in this case A to G. Data key switches 51 also correspond innumber to the individual electrodes with one key switch and one OR gatebeing assigned to a specific electrode.

Driver amplifiers operate to apply a potential +V to an associatedindividual electrode upon receipt of a pulse signal from a respective ORgate and in all other states to apply a ground, or other referencepotential to the individual electrode. 1

The driver amplifiers 61 and 62, connected to OR gates 40 and 41respectively, have their outputs connected across the pair of keep-aliveelectrodes 13.

The driver amplifier 63 has its output connected to the common terminal11 and its input connected to the output of the OR gate 42. Driveramplifiers 61, 62 and 63 operate in an identical manner to the driveramplifiers of bank 60.

Referring to FIG. 4a, 4b and 4c in conjunction with the circuits of FIG.1, the output clock pulses from oscillator 21 are shown in FIG. 4chaving a repetition rate of 50 microseconds. During the write cycle thepulses SW are applied to the selected individual electrodes and thepulses RE are applied to the common electrode. The width of the pulsesshown in FIGS. 40 and 4b are exaggerated; in reality they are each only0.5 microsecond in width as compared to the clock pulse spacing of 50microseconds. The combined pulses hav ing the width of l microsecondapplied alternately to both the selected individual electrodes and thecommon electrode will cause a discharge to take place between theselected individual electrodes and the common electrode. The relativelylong pulse width time allows the wall charge to build to a high enoughvalue to allow for the substitution of a sustaining pulse width signalto the electrodes in place of the wider discharge pulses. The keep-aliveelectrodes 13 continually receive the combined SW and RE pulses so adischarge continually exists in the area surrounding the keepaliveelectrodes which area is made to extend to the area of the individualand common electrodes.

Once the discharge (or entrance of data) takes place due to thedepression of selected ones of the keys associated with the switches 51the selected cell can be switched to a sustain cycle. This takes placewhen the selected switches 51 are opened. With switches 51 opened the Wpulses are not applied to the individual electrodes. The remaining Spulses on the individual electrodes have a pulse width of 0.5microsecond which is sufficient to maintain the discharge when coupledto the l microsecond width pulses RE applied to the common electrode.

in the erase cycle the key associated with the switch 52 is depressed,opening said switch and stopping the application of pulses R to thecommon electrode. The alternate application of E and S pulses to theindividual and common electrodes results in an insufficient build up ofthe wall charge and the discharges cease.

Although the present invention has been described in terms of apreferred embodiment it would be obvious to a person skilled in the artto extend the one character embodiment shown to a multi-character,multi-line display controller using the pulse width teachings of thepresent invention. It will also be obvious that many changes andmodifications may be made in the present invention without departingfrom the essential spirit of the invention. It is intended, therefore,in the annexed claims to cover all such changes and modifications thatfall within the true scope of the invention.

What is claimed is:

1. A discharge control circuit for use with a plasma gas cell of thetype having a pair of insulated electrodes positioned within the cellcomprising in combination:

a source of voltage pulses;

means for connecting said voltage pulses across said pair of electrodesin a reversing cycle; and

means for varying the pulse width of said pulses between differentfinite increments so as to vary the magnitude of the wall charge formedwithin said cell to thereby control the plasma discharge between saidpair of insulated electrodes.

2. A discharge control circuit for use with a plasma gas cell of thetype having a pair of electrode means conductively isolated from theplasma gas, comprising in combination:

means for providing pulsed signals;

means for varying between different finite increments of the effectivewidth of said pulsed signals so as to control the magnitude of the wallcharges formed in said cell; and

means for applying said pulsed signals to said pair of electrode means.

3. A discharge control circuit for use with a plasma gas cell of thetype having at least a pair of insulated electrode means conductivelyisolated from the plasma gas, comprising in combination:

a variable pulse width signal source;

means for applying pulsed signals from said source across pairs of saidelectrode means; and

means connected to said signal source for varying the width of thepulsed signals from said source between differentfinite increments so asto control the magnitude of the wall charge developed in said cell.

4. A circuit for controlling the occurrence of a discharge in a plasmagas cell of the type having insulated electrodes positioned within thecell, comprising in combination: I

a source ov voltage pulses;

means for alternately applying said voltage pulses across saidelectrodes; and

means for varying individually the pulse width of said voltage pulsesapplied alternately to said electrodes between different finiteincrements so as to control the magnitude of the wall charge formedwithin said cell, thereby controlling the occurrence of a plasmadischarge.

5. In combination:

a plasma gas cell;

a common electrode positioned in proximity to the plasma gas of saidcell;

a plurality of independent electrodes positioned in proximityto theplasma gas of said cell and opposing said common electrode;

keep-alive means for maintaining a plasma discharge in at least one areaof said plasma gas cell;

a variable pulse width signal source;

a plurality of switch means for independently connecting the pulsedsignals from said source to said plurality of independent electrodes;

means for delaying the pulsed signals from said source;

means for applying the delayed pulsed signals from said source to saidcommon electrode; and

means for selectively varying the pulse width of the pulse signals fromsaid signal source so as to control the magnitude of the wall chargedeveloped in said cell between the selected electrodes and said commonelectrode.

6. In combination:

a plasma gas cell;

a common electrode positioned in proximity to the plasma gas of saidcell;

a plurality of independent electrodes positioned in proximity to theplasma gas of said cell and opposing said common electrode;

keep-alive means for maintaining a plasma discharge in at least one areaof said plasma gas cell;

a source of pulsed signals;

means for delaying said pulsed signals for time periods corresponding tothe widths of said pulsed signals;

a plurality of gate means for receiving said undelayed pulsed signalsand for applying said signals to said plurality of independentelectrodes;

a plurality of independent switch means for connecting a delayed pulsesignal to an input of said gate means so as to cause a pulse signal ofincreased width to be applied to selected independent electrodes;

gate means for receiving at least two additionally delayed pulsedsignals from said delay means and for feeding the combined signal tosaid common electrode; and

switch means interposed in the path between one of said additionaldelayed pulsed signals and said gate means for discontinuing theapplication of one of the delayed pulsed signals to said commonelectrode so as to cause an erasure of discharges between saidindependent electrodes and said common electrodes when said independentswitch means are not connecting a delayed pulse to a gate means.

7. A discharge control circuit for use with a plasma gas cell of thetype having at least a pair of insulated electrode means conductivelyisolated from the plasma gas, comprising in combination:

a variable pulse width signal source;

means for applying pulsed signals from said source across pairs of saidelectrode means;

7 i 8 a plurality of delay means for receiving said pulsed means forcombining selected delayed pulsed signals signals and for delaying saidpulsed signals for time with the provided pulsed signals to form acomposperiods corresponding to the width of said pulsed ite pulsedsignal having the desired pulse width. signals; and

1. A discharge control circuit for use with a plasma gas cell of thetype having a pair of insulated electrodes positioned within the cellcomprising in combination: a source of voltage pulses; means forconnecting said voltage pulses across said pair of electrodes in areversing cycle; and means for varying the pulse width of said pulsesbetween different finite increments so as to vary the magnitude of thewall charge formed within said cell to thereby control the plasmadischarge between said pair of insulated electrodes.
 2. A dischargecontrol circuit for use with a plasma gas cell of the type having a pairof electrode means conductively isolated from the plasma gas, comprisingin combination: means for providing pulsed signals; means for varyingbetween different finite increments of the effective width of saidpulsed signals so as to control the magnitude of the wall charges formedin said cell; and means for applying said pulsed signals to said pair ofelectrode means.
 3. A discharge control circuit for use with a plasmagas cell of the type having at least a pair of insulated electrode meansconductively isolated from the plasma gas, comprising in combination: avariable pulse width signal source; means for applying pulsed signalsfrom said source across pairs of said electrode means; and meansconnected to said signal source for varying the width of the pulsedsignals from said source between different finite increments so as tocontrol the magnitude of the wall charge developed in said cell.
 4. Acircuit for controlling the occurrence of a discharge in a plasma gascell of the type having insulated electrodes positioned within the cell,comprising in combination: a source ov voltage pulses; means foralternately applying said voltage pulses across said electrodes; andmeans for varying individually the pulse width of said voltage pulsesapplied alternately to said electrodes between different finiteincrements so as to control the magnitude of the wall charge formedwithin said cell, thereby controlling the occurrence of a plasmadischarge.
 5. In combination: a plasma gas cell; a common electrodepositioned in proximity to the plasma gas of said cell; a plurality ofindependent electrodes positioned in proximity to the plasma gas of saidcell and opposing said common electrode; keep-alive means formaintaining a plasma discharge in at least one area of said plasma gascell; a variable pulse width signal source; a plurality of switch meansfor independently connecting the pulsed signals from said source to saidplurality of independent electrodes; means for delaying the pulsedsignals from said source; means for applying the delayed pulsed signalsfrom said source to said common electrode; and means for selectivelyvarying the pulse width of the pulse signals from said signal source soas to control the magnitude of the wall charge developed in said cellbetween the selected electrodes and said common electrode.
 6. Incombination: a plasma gas cell; a common electrode positioned inproximity to the plasma gas of said cell; a plurality of independentelectrodes positioned in proximity to the plasma gas of said cell andopposing said common electrode; keep-alive means for maintaining aplasma discharge in at least one area of said plasma gas cell; a sourceof pulsed signals; means for delaying said pulsed signals for timeperiods corresponding to the widths of said pulsed signals; a pluralityof gate means for receiving said undelayed pulsed signals and forapplying said signals to said plurality of independent electrodes; aplurality of independent switch means for connecting a delayed pulsesignal to an input of said gate means so as to cause a pulse signal ofincreased width to be applied to selected independent electrodes; gatemeans for receiving at leAst two additionally delayed pulsed signalsfrom said delay means and for feeding the combined signal to said commonelectrode; and switch means interposed in the path between one of saidadditional delayed pulsed signals and said gate means for discontinuingthe application of one of the delayed pulsed signals to said commonelectrode so as to cause an erasure of discharges between saidindependent electrodes and said common electrodes when said independentswitch means are not connecting a delayed pulse to a gate means.
 7. Adischarge control circuit for use with a plasma gas cell of the typehaving at least a pair of insulated electrode means conductivelyisolated from the plasma gas, comprising in combination: a variablepulse width signal source; means for applying pulsed signals from saidsource across pairs of said electrode means; a plurality of delay meansfor receiving said pulsed signals and for delaying said pulsed signalsfor time periods corresponding to the width of said pulsed signals; andmeans for combining selected delayed pulsed signals with the providedpulsed signals to form a composite pulsed signal having the desiredpulse width.